This invention relates generally to synchronizing (sync) signal recovery systems and particularly to a digitally based synchronizing signal recovery system that is noise immune.
Synchronizing signal recovery systems are well known in the art, especially in connection with the recovery of synchronizing signals in transmitted television signals. With the advent of digital techniques and countdown circuits for developing synchronizing signals, new levels of accuracy and noise immunity are available in television receiver design. Additionally, the rise of computer technology and high resolution cathode ray tube display systems has fueled the need for synchronizing signal systems that operate on more than one frequency. These are generally referred to as multi-frequency or multi-standard synchronizing signal systems. Multi-frequency systems generally operate on the basis of developing a multiple or submultiple of a given clock frequency for generating appropriate synchronizing signals. It is of course imperative that any synchronizing system be capable of control or adjustment by the synchronizing signals in the received signal to assure that information is properly displayed. Thus it is standard to utilize a closed loop control system, such as a phase locked loop, for assuring that the regenerated sync pulses coincide with the syncs in the transmitted signals.
The present invention is directed to a multi-standard synchronizing signal recovery system that adjusts to the synchronizing pulses in the incoming signal, but is not frequency dependent. The system of the invention provides a high level of noise immunity since it looks for synchronizing pulses expected to occur in a narrow time window, which therefore increases the system noise immunity. On the other hand, the window is widened when seeking sync pulses, or when signal changes occur, for example, during a channel change in the television receiver. The system will also produce periodic output syncs in the absence of expected incoming syncs within the window for a time period depending upon the history of the received signal and produce minimum frequency (default signals) output sync in the absence of an incoming sync signal. With the system of the invention, sync drop outs because of noise impulses and the like are minimized.